Information storage devices are used to retrieve and/or store data in computers and other consumer electronics devices. A magnetic hard disk drive is an example of an information storage device that includes one or more heads that can both read and write, but other information storage devices also include heads—sometimes including heads that cannot write. For convenience, all heads that can read may be referred to as “read heads” herein, regardless of other devices and functions the read head may also perform (e.g. writing, micro-actuation, flying height control, touch down detection, lapping control, etc). A contemporary trend is to include more devices in and on the head, which may facilitate reading and writing (for example, a heater to reduce head-disk spacing during reading or writing), and/or perform other functions such as microactuation or lapping control. As more devices are included in and on the head, the number of electrical connections to the head must increase. Hence there is a need in the art for methods and structures to facilitate or accommodate an increased number of electrical connections to a head in an information storage device.
In a modern magnetic hard disk drive device, each head is a sub-component of a head-gimbal assembly (HGA) that typically includes a suspension assembly with a laminated flexure to carry the electrical signals to and from the head. The HGA, in turn, is a sub-component of a head-stack assembly (HSA) that typically includes a plurality of HGAs, an actuator, and a flexible printed circuit (FPC). The plurality of HGAs are attached to various arms of the actuator.
Modern laminated flexures typically include conductive copper traces that are isolated from a stainless steel structural layer by a polyimide dielectric layer. So that the signals from/to the head can reach the FPC on the actuator body, each HGA flexure includes a flexure tail that extends away from the head along a corresponding actuator arm and ultimately attaches to the FPC adjacent the actuator body. That is, the flexure includes traces that extend from adjacent the head and continue along the flexure tail to electrical connection points. The FPC includes conductive electrical terminals that correspond to the electrical connection points of the flexure tail.
To facilitate electrical connection of the conductive traces of the flexure tails to the conductive electrical terminals of the FPC during the HSA manufacturing process, the flexure tails must first be properly positioned relative to the FPC so that the conductive traces of the flexure tails are aligned with the conductive electrical terminals of the FPC. Then the flexure tails must be held or constrained against the conductive electrical terminals of the FPC while the aforementioned electrical connections are made (e.g. by ultrasonic bonding, solder jet bonding, or solder bump reflow).
However, recently for some disk drive products, the aforementioned electrical connections may employ a type of anisotropic conductive film (ACF) bonding. An anisotropic conductive film is typically an adhesive doped with conductive beads or cylindrical particles of uniform or similar diameter. As the doped adhesive is compressed and cured, it is squeezed between the surfaces to be bonded with sufficient uniform pressure that a single layer of the conductive beads may make contact with both surfaces to be bonded. In this way, the thickness of the adhesive layer between the bonded surfaces may become approximately equal to the size of the compressed conductive beads. The cured adhesive film may conduct electricity via the contacting beads in a direction normal to the bonded surfaces (though may not necessarily conduct electricity parallel to the bonded surfaces, since the beads may not touch each other laterally—though axially each bead is forced to contact both of the surfaces to be bonded—hence the term “anisotropic”).
Maintaining sufficient uniform pressure during adhesive curing, such that a single layer of conductive beads in an ACF makes contact with both opposing surfaces to be bonded, may be achievable for existing HGA designs using a patterned thermode tool that is aligned to press only upon bond pad locations. However, in a high-volume manufacturing environment like that necessitated by the very competitive information storage device industry, there is a practical requirement for fast, cost-effective, and robust bonding of many bond pads simultaneously, without the need for precise alignment of the thermode tool. Hence, there is a need in the art for a flexure tail design that can receive pressure only at the bond pad locations from a non-patterned thermode tool that does not need to be precisely aligned.
Some previously disclosed designs have included discontinuous islands in the structural layer of the flexure tail, to facilitate simultaneous ACF bonding of many bond pads simultaneously by a non-patterned thermode tool. However, such discontinuous islands can be disadvantageous in the event that rework of the flexure tail bonding process becomes necessary. Specifically, if a flexure tail needs to be removed after bonding to the FPC, a flexure tail weakness due to the discontinuous nature of the structural layer islands may encourage the flexure tail to tear, rather than to peel away thoroughly from the FPC. Such tearing may result in a portion of the unwanted flexure tail to remain bonded to the FPC, which may then interfere with the adhesion and the proper electrical connection of a replacement flexure tail to be bonded to the same portion of the FPC.
Accordingly, there is a need in the art for an improved flexure tail design that facilitates the use of a non-patterned thermode tool to simultaneously apply an acceptably uniform pressure to a group of bond pads during HSA manufacture (e.g. during an ACF bonding process), and that has an enhanced peel strength for possible HSA rework.